1. Field of the Invention
The present invention relates to a method of controlling the developer toner density of a developing device such as an electrophotographic reproducing machine.
2. Description of the Prior Art
As the method of controlling the developer toner density of a developing device using a two-component developer container toner and magnetic carrier, there has been known in the prior art a method of supplying the toner to the developing device in a manner to hold at a constant level the output of a toner sensor for detecting the toner density (magnetic permeability) variations in terms of the magnetoresistance variations of the developer. In case continuous copying operation are performed by the electrophotographic reproducing machine using such a developing device, the developer toner density gradually drops up to 100 copies, as indicated by a broken curve in FIG. 4, despite of the fact the toner sensor output is held at a constant level. This invites a phenomenon that the copy density accordingly drops.
According to an inventor's investigations, the main cause for that phenomenon is as follows. If the development is continued in the continuous copying operations by the developing device, the charges of the toner will gradually accumulate until they become saturated after an agitation period of 5 to 10 minutes or in a copy number of 100 sheets, because the developer in the developing device is agitated during the continuous copying operations by the agitation roller. The more the charges increase, the more the electric repulsions between the toner particles will increase to enlarge the gaps between the carrier particles. As a result, the magnetic resistance of the developer increases so that the output of the toner sensor varies similarly to the case in which the toner density increases. In order to prevent the reduction of the toner density for that cause, it has also been investigated to be effective that a level at which the output of the toner sensor has to be held constant is gradually varied in the direction to increase the toner density until the charges of the toner become saturated. Since the toner charges will not abruptly attenuate even if the development or agitation is interrupted, they may be made to become saturated in terms of the integrated development period or copy number, assuming that an identical charged state is held within the interruption of a constant period. It is therefore sufficient that the varied ouput level of the toner sensor to be held may be returned back in case the interruption period exceeds a constant value.